[PATCH v4 7/8] vhost: Add new UAPI to support change to task mode

[Cindy Lu <lulu@redhat.com>]

Add a new UAPI to enable setting the vhost device to task mode.
The userspace application can use VHOST_SET_INHERIT_FROM_OWNER
to configure the mode if necessary.
This setting must be applied before VHOST_SET_OWNER, as the worker
will be created in the VHOST_SET_OWNER function

Signed-off-by: Cindy Lu <lulu@redhat.com>
---
 drivers/vhost/vhost.c      | 22 +++++++++++++++++++++-
 include/uapi/linux/vhost.h | 18 ++++++++++++++++++
 2 files changed, 39 insertions(+), 1 deletion(-)

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 3e9cb99da1b5..12c3bf3d1ed4 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -2257,15 +2257,35 @@ long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *argp)
 {
 	struct eventfd_ctx *ctx;
 	u64 p;
-	long r;
+	long r = 0;
 	int i, fd;
+	u8 inherit_owner;
 
 	/* If you are not the owner, you can become one */
 	if (ioctl == VHOST_SET_OWNER) {
 		r = vhost_dev_set_owner(d);
 		goto done;
 	}
+	if (ioctl == VHOST_SET_INHERIT_FROM_OWNER) {
+		/*inherit_owner can only be modified before owner is set*/
+		if (vhost_dev_has_owner(d)) {
+			r = -EBUSY;
+			goto done;
+		}
+		if (copy_from_user(&inherit_owner, argp, sizeof(u8))) {
+			r = -EFAULT;
+			goto done;
+		}
+		/* Validate the inherit_owner value, ensuring it is either 0 or 1 */
+		if (inherit_owner > 1) {
+			r = -EINVAL;
+			goto done;
+		}
+
+		d->inherit_owner = (bool)inherit_owner;
 
+		goto done;
+	}
 	/* You must be the owner to do anything else */
 	r = vhost_dev_check_owner(d);
 	if (r)
diff --git a/include/uapi/linux/vhost.h b/include/uapi/linux/vhost.h
index b95dd84eef2d..d7564d62b76d 100644
--- a/include/uapi/linux/vhost.h
+++ b/include/uapi/linux/vhost.h
@@ -235,4 +235,22 @@
  */
 #define VHOST_VDPA_GET_VRING_SIZE	_IOWR(VHOST_VIRTIO, 0x82,	\
 					      struct vhost_vring_state)
+
+/**
+ * VHOST_SET_INHERIT_FROM_OWNER - Set the inherit_owner flag for the vhost device
+ *
+ * @param inherit_owner: An 8-bit value that determines the vhost thread mode
+ *
+ * When inherit_owner is set to 1:
+ *   - The VHOST worker threads inherit its values/checks from
+ *     the thread that owns the VHOST device, The vhost threads will
+ *     be counted in the nproc rlimits.
+ *
+ * When inherit_owner is set to 0:
+ *   - The VHOST worker threads will use the traditional kernel thread (kthread)
+ *     implementation, which may be preferred by older userspace applications that
+ *     do not utilize the newer vhost_task concept.
+ */
+#define VHOST_SET_INHERIT_FROM_OWNER _IOW(VHOST_VIRTIO, 0x83, __u8)
+
 #endif
-- 
2.45.0